1. Field of the Invention
This invention relates to chromium electrodeposits, and, particularly, to chromium electrodeposits which contain a multitude of microporous areas over substantially the entire surface area thereof.
2. Description of the Prior Art
Microporous or microdiscontinuous chromium electrodeposits are known to exhibit significantly improved corrosion resistance as compared to conventional chromium electrodeposits. In such electrodeposits, corrosion occurs at each point of discontinuity, which cannot be resolved by the unaided eye. Accordingly, microporous chromium coatings are particularly useful for decorative purposes.
Unfortunately, conventional chromium electrodeposits do not form a sufficient number of micropores to be considered an effective microporous surface. Therefore, it has been necessary to provide a suitable under-deposit on the basis metal which contains a plurality of solid particles therein. The chromium layer then assumes a microporous condition on the areas where the solid particles are present. These processes are represented by the following prior art references: U.S. Pat. Nos. 3,428,441; 3,449,223 and 3,595,762.
For example, Odekerken, in U.S. Pat. No. 3,449,223, describes a process of forming a microporous decorative chromium electrodeposit by first plating a basis metal with nickel from a bright nickel electroplating bath containing insoluble solid particles. The bright nickel coating thusproduced contains solid particles therein, which provide the chromium layer with micropores above those areas where solid particles are present.
Similarly, Chessin, in U.S. Pat. No. 3,595,762, describes an improved method and article in which two undercoat layers are provided, the first being a stratum of suitable particles formed by dipping the basis metal into a fluidized bed of such particles. The second layer thereon is an electrodeposited, conductive metal layer free of said particles, e.g. a layer containing sulfur-free nickel, which fixes the particles in the layer beneath it. The chromium deposit then is formed on the conductive layer having the particles embedded therein, thus forming the desired microporous chromium electrodeposit.
Both the above described processes, however, require one or more undercoat layers with solid particles therein. Accordingly, it would be advantageous if the chromium electrodeposit could be formed directly on the basis metal without the necessity of providing a particular undercoat layer with particles therein.
Odekerken U.S. Pat. No. 3,282,810, Examples 1 and 2 describe a method of forming microporous chromium layers. However, in order to bring about this microporous structure, use is made of colloidal particles as found in an emulsion, such colloidal particles having a particle size between 0.01 and 0.1.mu.. These particles are present in either the underlying nickel layer or in the chromium layer itself.
In view of the foregoing, it is an object of the present invention to provide an improved microporous chromium article.
Another object herein is to provide a method of making such improved microporous chromium article.
Still a further object of the invention is to provide a chromium plating bath from which microporous chromium electrodeposits can be formed directly onto a basis metal.
Yet another object of the invention is to provide a bright, non-iridescent, non-blue haze, microporous decorative chromium electrodeposited article without intermediate layers or the chromium layer itself containing solid particles therein.
Among the other objects of the invention is to provide a chromium plating bath having high throwing power and nickel activation without producing a cloudy chromium deposit known as "white wash" which is commonly encountered when attempting to plate chromium on passive nickel surfaces.